Isolated DC/DC (direct current/direct current) converters can have zero voltage switching (ZVS), or zero current switching (ZCS), making it possible to reduce the switching losses during voltage conversion. These converters are thus particularly advantageous in a car application where the energy resource is limited. In a vehicle, one can use a voltage converter to adapt the voltage levels between several electrical networks of the vehicle or to convert a voltage between an energy source and an electrical consumer on board the vehicle.
An isolated DC/DC voltage converter is known from U.S. Pat. No. 5,754,413 and illustrated in FIG. 1. The converter comprises two switches in a half-bridge arrangement, which are connected at their midpoint to a branch which comprises two series-connected transformers. The switches control the transmission of energy through the transformers in order to achieve conversion of an input voltage of the converter into an output voltage. Diodes connected to the secondary windings of the transformers make it possible to rectify the output signal. The output voltage is obtained by controlling the duty ratio of the switches. By modifying the duty ratio to achieve a target output voltage value, one adjusts the gain of the converter in order to reach the target value of the output voltage. In particular, when the input voltage of the isolated DC/DC converter varies, it is known to vary the duty ratio of the switches of the isolated DC/DC converter in order to regulate its output voltage, that is to say to maintain its output voltage at a desired value.
However, the voltage stress of the rectifying diodes is a function of the duty ratio of the switches of the converter. This stress can become high, when the duty ratio approaches 0% or 100%. To limit the voltage stress of the rectifying diodes, respective transformation ratios that are different for the two transformers are provided. However, this complicates the design of the converter, since the transformers cannot be identical, and the current in the secondary winding presents discontinuities.
In addition, by working with a variable duty ratio, the ripples of the output current can vary strongly, resulting in variation of the yield of the converter. To preserve operation with good yield, the variability of the duty ratio must be low. However, in a vehicle, the voltage of an energy source such as a battery can vary greatly depending on the available energy. Such a variation at the input of the converter involves varying the duty ratio correspondingly, which limits the use of the isolated DC/DC converter in a vehicle.